Advances in solar cell manufacturing require some kind of patterning of the doped regions, instead of the blanket diffusions that are now commonly used throughout the solar industry. For example, for a better front-side emitter performance with a lower amount of recombination losses in a standard silicon solar cell, one requires a higher doping underneath the metal contact area than in the non-contact area.
One method for patterning, used in combination with ion implantation, is known from WO2011/152982 A2. Herein, the patterning of doped regions is based on the use of shadow-masks that are installed inside the ion implanter. These masks can prevent or lower the exposure of certain parts of the solar wafers to the ion beam. After annealing the wafers will have a sheet resistance profile that has some kind of pattern along the wafer surface.
The technique has a disadvantage in that the installation and maneuvering of shadow masks inside the implanter adds to the complexity of the machine and therefore adds significant cost to the machine sales price. Through depreciation the additional machine complexity then adds to the cost of the produced solar cells.
Another disadvantage is that these shadow masks heat up quite a bit during continuous use in an ion implanter and will therefore generally change shape to some extent. For this reason there is a limit to how fine one can make patterns in this way. This is also important if one needs multiple shadow masks that have to have a certain alignment from one mask to the next. This situation occurs for instance when one is using a high-efficiency solar cell design called inter-digitated back-contact (IBC). In this design one places p- and n-type doped regions side-by-side in a complex inter-digitated pattern on the rearside of the wafers.
Another disadvantage of shadow masking is that the masks themselves cannot make any arbitrary shape because the mask needs to be able support itself. For this reason, the patterns made on the semiconductor substrates are limited in their design.